Propellers for multiple propeller ships



y 23, JL KREITNER ETAL, 254,286

PROPELLERS FOR MULTIPLE PROPELLER SHIPS Filed June 6, 1940 INVENTORS Jail/MN HEE/TNEQ 205E 27 M. KEISTAL ATTORNEY Patented May 23,1944

I UmTEDSTATES PATENT OFFICE,

Johann Kreitner, Brooklyn, and Robert M. KristaL'New York, N. Y.

Applicationjlunc 6,1940, Serial Nat 391E861 a "Glaims." (c1. lit-37) This invention relates to ship propellers and more particularly to multiple propellerships.

Heretofore, multiple propeller-ships were fitted with propellers of the same number of blades on propellers of each symmetrically arrangedpair. This arrangement was instrumental in creating a particularly unpleasant form of vibration, namely fluctuations between comparative silence and maximum shaking, caused by imperfect synchronism between the line shafts.

It is these fluctuations which contribute greatly to the discomfort of those aboard the ship, while steady monotonous vibrations of the same average intensity are almost-unnoticeable as experience with single propeller ships shows. It is therefore an object of the present inven tion to eliminate the fluctuating vibrations of multiple propeller ships thus adding to the pleas-I ure and comfort of passengers. v It is a further object, of the invention toelim -r inate the consequences of imperfect synchronism, namely, the periodical alternation between symmetrical and asymmetrical blade positions of paired propellers. v J} I These and other objectsand advantages ofthe invention and themeans for their attainment will be more apparent from the following description, taken in connection with the accompanying drawing, illustrating some embodiments Qby which the invention may be realized, and in which: 1 f V .Fig. 1 is anendelevation showing an embodiment of the invention in a twin screwship. I A Figs. 2 and3 are similar end views of a quadruple screwship showing two difierent embodi ments of 'my invention. It is a known fact that fluctuating vibrations in multiple propeller shipsare caused by alternatingly synchronous and anti-synchronous oscillation of the hydraulic forces,,acting .upon the blades of the propellers and upon nearby structures of the hull. I f H A propeller blade during one revolution passes through homogenous layers of water; thus the hydraulic thrust upon the blades varies within wide limitsf I'n'particular, a strong increase in force occurs when a blade passes through the dead boundary layer close to the skin. This is repeated when the following blade reaches the same position. Hence the total force produced by one propeller of any blade type oscillates periodically as the number of blades times the number of revolutions.

Where paired propellers of the same number of blades of a multiple propeller ship run in opposite directions in perfect symmetry--that is with blades passing the vertical position at tl 1e same moment-the longitudinal and vertical components of the resultant force acting on the ship oscillate according to the sum of the oscillations of the paired propellers.

But exact symmetry cannot continue for any length of time with two independent sets of propulsive machinery. If one enginerevolves faster by only a fraction of one revolution per minute, it will be but a short time before the propellers will not rotate symmetrically; after a score of seconds or so one propeller may be ahead of the other by one-half of the angle between two blades, or 45 with three or four bladed Wheels of the screwpropeller type); the oscillations which act upon the two propellers are then in approximately opposite phase or antisynchronous, thus. nullifying the resulting OSCil? lation andthe ship becomes almost quiet. But after another similar period a symmetrical position is reached once more causing the vibrations to pitch up to a maximum. I Vibration itself does not annoy passengersso much as the irregularity of its intensity. The slow fluctuation is particularly annoying because passengers become conscious of the extreme contrast between silence and shaking and nervously await the recurring. maximum shaking.

Ship engineers usually attempt to overcome these fluctuations by running the paired engines at' different R. P. M. in order to shorten the periods of fluctuation. However, this has decided disadvantages in that this practice throws a difierent load .on port and starboard engine respectively, necessitating constant helm compensation in order .to keep in course. which effects economy of operation adversely. Furthermore in order to be really effective the difference required would be 10 R. P. M. or more, which is impractical from an economic viewpoint. Ship designers too, have attempted to eliminate these fluctuating vibrations by keeping the pairse of propulsive machinery mathematically synchronous. This was sought to be accomplished by mechanical coupling of the shafts (which is impractical with large units) by electrical coupling of the shafts (which necessitates considerable additional weight and cost, except in the special case of electric drive by A. C.

synchron motors; by arrangements for keeping the two drives in step by special governors on the propulsive machinery (which however is ineffective in maintaining sufiiciently exact synchronism under real service conditions because wave and rudder action constantly interfere with the propeller load).

The present invention overcomes all of these difliculties by using paired propellers of different number of blades, for example, a three bladed propeller on portside and a four bladed propeller on starboard Thus one propeller of a pair of creates a total force oscillating as R. P. M. 3, while the resultant force of the other propeller oscillates as R. P. M. 4.

Our invention thus completely does away with any synchronism of blade action and with the ensuing fluctuating vibrations.

In the case of quadruple propeller drive each propeller should be of a difierent number of blades, or at least the propellers of each symmetrically arranged pair should have a different number of blades in order to avoid synchronism of forces acting symmetrically upon closely adjacent parts of the hull. By such an arrangement, shown in Figs. 2 and 3, propellers of the same number of blades are not symmetrically placed and are of such distance apart as to obviate practically all effects of synchronism.

One embodiment of our invention comprises paired propellers of different number of blades so designed that each propeller consumes substantially the same power at the same R. P. M.

and speed, so that identical sets of propulsive machinery can be'used in spite of the difference in number of propeller blades. The influence of the different number of blades may be compensated by a minor difference in the geometric pitch.

Instead of varying the geometric pitch to compensate for the difference in number of blades, a slight difierence in diameter may be employed to attain the same result.

In Fig. 1 there is shown a conventional hull form H provided with two screw propellers. These propellers are symmetrically arranged, the portside propeller l8 having three blades and the starboard propeller H having four blades. The propellers as shown, are substantially the same diameter and have approximately the same blade area, the geometric pitch varying slightly in order to compensate for the influence of the different number of blades on power consumption. Such compensation may also be achieved by varying the diameter slightly.

In Figs. 2 and 3, a quadruple screw ship is shown, I2 and [5 indicating the forward pair of b of propellers is approximately one-twelfth of the ships length.

In Fig. 3 the propellers of the same number of blades are arranged on the same side of the ship.

Thus both arrangements obviate any symmetry and thereby greatly reduce the possible effects of synchronism.

A complete elimination of any possible synchronism may be achieved in any ship by using a different number of blades for each individual propeller. However, since ship building practice, so far, is reluctant to accept other than three or four bladed screw propellers, the quadruple screw arrangements shown in the drawing are eliminated to these number of blades.

This invention refers not only to screw propellers but to any type of propeller wherein individual blades are used for propulsion.

Although the drawing, and the above speciflcation disclose the best modes in which we have contemplated embodying our invention, we desire in no way to be limited to the details of such disclosure, for in the further practical application of our invention many changes in the forms and proportions may be made'as circumstances require or experience suggests Without departing from the spirit of the invention Within the scope of the appended claims.

Having thus described our invention what we claim as new and desire to secure by Letters Patent is:

1. In a power-propelled water craft having a hull constructed to float in the-water, the combination with said hull, of a plurality of power driven propellers spaced transversely on opposite sides of the center of said hull and symmetrically arranged relative thereto, each of said propellers having individual blades, the propeller on one side of the center of said hull hav ing a different number of blades from the rcorrespondingly arranged propeller on the opposite side of the center thereof.

2. In a power-propelled water craft havingia hull adapted to be partially submerged in the water, the combination with said hull, of a-plurality of power driven propellers spaced transversely on opposite sides of the center of said hull and symmetrically arranged relative thereto in position to operate below the surface ofthe water, each of said propellers havingradiating blades of the screw type, the propeller on"one side of the center of said hull having a different number of blades from the symmetrically arranged propeller on the opposite side of, the center thereof.

3. In power-propelled water craft having a body constructed to float in water, means for propelling said body through said water, said means comprising at least two propellers syme metrically disposed on opposite sides of the longitudinal medial line of said body, the prof peller on one side of said medial line having} different number of blades from the other *syn'if metrically arranged propeller on the opposite id JOHANN*KREITNER,."" ROBERT M. KRISTAL- 

